Generally when rolling stats, you can get enough points so that your character doesn't really have any weaknesses. But for classes that use a lot of stats or for some dual-class and multiclass combinations, there's a little more pressure to get the most out of every stat point.
Unlike later editions of DnD that give a +1 to something for every 2 points, 2nd Edition does not use a uniform system for stat benefits. You can look in the Melee & Magic manual for these, though I will include the pertinent numbers here.
However, the manual doesn't include the effects of Constitution on high-level characters, which I have put below.
It also doesn't have the permanent stat increases you will come across in BG, BG2 and TOB. They are below in the spoiler box.
This guide will sometimes refer to class archetypes. Here they are:
Warriors: Fighter, Paladin, Ranger, Monk, Barbarian, Blackguard
Wizards: Mage, Sorcerer
Priests: Cleric, Druid
Rogues: Thief, Bard
+1 to all stats in the form of a tome for each scattered throughout the game. However, there are three such tomes for Wisdom if you play through BG and its Durlag's Tower area.
If you export your character, import them to a new game and reacquire the tomes, the bonuses are cumulative.
These can be used by party members, though obviously that bonus would go away on a new playthrough.
Baldur's Gate 2: Shadows of Amn
A dream will force you to choose between Strength, Dexterity, Constitution or Wisdom. You will lose -1 from that stat.
Right before the final encounter of the game, you will get to make a series of choices. In each, there is a good choice and an evil choice. If you choose any of the evil choices, your alignment will change to Neutral Evil, and there will be no way of changing it back. Choosing the good choices has no effect on your alignment.
I'm not sure what BG2EE will do, but in BG2's original release, this has no effect on Paladin abilities. The exception is Turn Undead, which controls undead rather than destroys them if the Paladin or Cleric is evil. It would be kind of cool if, in BGEE2, this changes a Paladin into a Blackguard, though. Time will tell.
Good: Immunity to +1 or normal weapons
Evil: +2 Constitution
Good: +10 resistance to magic (you will lose 2 max HP, some XP and 1 dexterity in the process)
Evil: -2 bonus to AC
Good: -2 bonus to all saving throws
Evil: +15 to HP
Good: +20% resistances to fire, cold and electricity
Evil: Gain 200,000 experience
Good: +1 to Wisdom and Charisma
Evil: +2 to Strength (may also give +1 to primary stat, may or may not stack with the +2 str)
Baldur's Gate 2: Throne of Bhaal
The Machine of Lum the Mad will give you (or any character in your party you choose) +1 to each stat.
The Deck of Many Things can give you +1 to your class' primary stat. You'll have to either get lucky or reload saved games until you get it (look up a guide on how -- you can't get all "good" draws no matter how much you reload). You get Strength for warriors, Intelligence for wizards, Wisdom for priests, and Dexterity for rogues.
Dual-classed characters get the stat for the active class. For multi-classed characters, it prioritizes Intelligence > Wisdom > Strength > Dexterity. So a Cleric/Mage gets Intelligence and a Fighter/Cleric gets Wisdom, for example.
Now for the bonuses you get for certain stat thresholds.
Note that in 2nd Edition, stats have no bearing on your saves (with the exception of Constitution helping the short races) or how powerful or difficult to resist your spells are. There are also minor bonuses for each stat that I'm not bothering to list here: things like carrying capacity and lore.
I count up to either the point where benefits cap or at the highest natural score attainable in a single playthrough (no buffs from magic items or spells). That number may not be possible for all alignments, races and classes. See the spoiler for details.Strength
16: +1 damage
17: +1 to hit, +1 damage
18: +1 to hit, +2 damage
19: +3 to hit, +7 damage
20: +3 to hit, +8 damage
21: +4 to hit, +9 damage
22: +4 to hit, +10 damage
23: +5 to hit, +11 damage
24: +6 to hit, +12 damage
NOTE: Warriors with 18 strength get exceptional strength ranging from 1 to 100 (shown as 00). Don't worry too much about it though, because 18/51 through 18/99 give +2 to hit, and when you eventually use the tome, 19 is better than 18/00.Dexterity
15: -1 AC
16: -2 AC, +1 to hit ranged
17: -3 AC, +2 to hit ranged
18: -4 AC, +2 to hit ranged
19: -4 AC, +3 to hit ranged
20: -4 AC, +3 to hit ranged
21: -5 AC, +4 to hit ranged
22: -5 AC, +4 to hit rangedConstitution
15: +1 hp/level for first 9 or 10 levels
16: +2 hp/level for first 9 or 10 levels
17: +3 hp/level for first 9 levels (warriors only)
18: +4 hp/level for first 9 levels (warriors only)
19: +5 hp/level for first 9 levels (warriors only)
20: +5 hp/level for first 9 levels (warriors only), regeneration
21: +6 hp/level for first 9 levels (warriors only), regeneration
22: +6 hp/level for first 9 levels (warriors only), regeneration
23: +6 hp/level for first 9 levels (warriors only), regeneration
NOTE: Bonus HP is retroactive when Constitution is gained or lost, and bonus saves for the short races are recalculated on level-up.
NOTE 2: Bonus HP per level only apply to the first 9 levels for warriors and priests; 10 for rogues and wizards. After that, classes gain a set (and very small) amount of hit points per level.
NOTE 3: For every point of Constitution above 20, regeneration (automatic healing over time) is slightly faster. It will still be too slow to make a difference in combat, however. It's more of a convenience. There are also a few items in BG2 and TOB which grant regeneration, though not many.Intelligence
9: 6 max known arcane spells/spell level
10: 7 max known arcane spells/spell level
11: 7 max known arcane spells/spell level
12: 7 max known arcane spells/spell level
13: 9 max known arcane spells/spell level
14: 9 max known arcane spells/spell level
15: 11 max known arcane spells/spell level
16: 11 max known arcane spells/spell level
17: 14 max known arcane spells/spell level
18: 18 max known arcane spells/spell level
NOTE: Arcane spells are cast by wizards and Bards.
NOTE 2: Intelligence also affects the chance to learn a spell from scrolls, but this is irrelevant on normal and lower difficulty, as success is guaranteed.
NOTE 3: Because Sorcerers can only learn the spells they select on level-up, Intelligence means nothing special to them. Unlike in later editions of DND, Charisma doesn't mean anything special to them, either.
NOTE 3: Intelligence does not give magic users additional spells per day.Wisdom
13: Extra 1st-level spell/day
14: Extra 1st-level spell/day
15: Extra 2nd-level spell/day
16: Extra 2nd-level spell/day
17: Extra 3rd-level spell/day
18: Extra 4th-level spell/day
19: Extra 1st- and 4th-level spell/day
20: Extra 2nd- and 4th-level spell/day
21: Extra 3rd- and 5th-level spell/day
22: Extra 4th- and 5th-level spell/day
23: Extra 5th- and 5th-level spell/day
24: Extra 6th- and 6th-level spell/day
NOTE: Bonuses are cumulative and divine magic user only. A priest, Paladin or Ranger with 14 Wisdom gets two additional 1st-level spells.
NOTE 2: Your Wisdom score can effect what options are available to you in the Limited Wish and Wish spells in BG2. These are cast by Mages and Sorcerers.Charisma
16: 5% discount at stores
17: 10% discount at stores
18: 15% discount at stores
19: 20% discount at stores
20: 25% discount at stores
NOTE: The above numbers are not in the manual, and I found them on a Wiki site
. There is no price penalty for low Charisma, and the discount caps at 25%.
NOTE 2: Charisma also affects the morale of your party (how likely they are to flee in combat) and your Reaction Adjustment. A combination of your Reputation and Charisma, this can mean a better or worse reward for doing a quest, and some NPCs will refuse to join your party if your Reaction Adjustment is too low.
For example, only a Lawful Good character (therefore starting with 12 Reputation) with 18 Charisma will be given a Dagger +1 for doing a particular quest in Candlekeep. Otherwise, you are rewarded with a few gold.
While I've taken great care to ensure this post is error-free, someone will probably find something to correct and embarrass me terribly. Sorry in advance. I will follow this thread and update as needed.@Wiggles
three comments down recommends an Android smartphone app with much of this information and more.